In general, an electric power steering device is configured to bias a worm shaft toward a worm wheel side in order to reduce backlash of a worm reduction gear. As the worm shaft is tilted around a bearing that is provided on an electric motor side, axial misalignment is caused at the position where the worm shaft and an output shaft of the electric motor are coupled. A joint that permits the axial misalignment and transmits rotational torque of the electric motor to the worm shaft becomes necessary.
As a shaft coupler having the joint of this kind, WO2005/116473 discloses a shaft coupler including a coupling base body that is coupled to a rotating shaft, a coupling base body that is coupled to a steering shaft, and a pair of rotation transmission members that is arranged between the rotating shaft and the steering shaft via the coupling base bodies and that transmits rotation of the rotating shaft to the steering shaft, in which each of the rotation transmission members includes an annular base part and a pair of projection parts that is provided on the base part in an integrated manner to be projected in the radial direction.